When uploading survey data, users can choose a threshold that suits their accuracy needs. Selecting the appropriate thresholds will help ensure the outputs generated align with the user’s expectations.
What is Ground Control Point (GCP) error?
After you submit your survey, it enters our processing pipeline, where a complex feature-matching process occurs. Here, an algorithm detects similar points in overlapping photos and creates a sparse point cloud. A computer will georeference the sparse point cloud by marking ground control points (GCPs) in the source imagery, which adjusts the model. A geospatial specialist will intervene if the computer doesn’t mark any points, and those points will be manually marked.
If the “Skip all QA” option is selected, points that cannot be marked by a computer will be excluded from processing. No geospatial specialist will be assigned to manually mark GCPs.
After the point cloud has been georeferenced, a root mean square error (RMSE) calculation is performed to get the distance between the original input position of the GCP and the estimated position of the same GCP after the adjustment is made. This distance is the GCP error.
Strict, Moderate, Lenient, or Skip all QA
The user’s choice—Strict, Moderate, Lenient, or Skip all QA—will determine what RMSE value a survey will be flagged and placed on hold. The higher the allowable GCP error, the more likely the survey will flow through our processing pipeline without delays.
The RMSE thresholds are listed below.
|5 cm (0.16 ft)
|10 cm (0.33 ft)
|20 cm (0.66 ft)
|Skip all QA
|No data review. Users can review GCP errors in the Processing Report.
A geospatial specialist will perform additional photogrammetry troubleshooting if any GCP has an RMSE above the set threshold. If the error is not resolved, a geospatial specialist will place the survey on hold and contact the user who uploaded it with an explanation for:
Why this is occurring, and
Alternative options for processing
Propeller will not continue processing a survey where a single GCP exceeds the RMSE threshold without customer permission.
What about comparing to past surveys?
The final step of the Propeller processing pipeline is to compare the submitted survey with past overlapping surveys.
When the second box is checked and Strict is selected, if more than 50% of the survey area has changed by more than 0.10 m (0.33 ft), the survey is flagged and requires the intervention of a geospatial specialist. If the changes appear to be due to legitimate site activity, the geospatial specialist will release the survey. If the changes are not due to site activity and are observed in areas that appear not to have changed, then the geospatial specialist will place the survey on hold and reach out to the user who uploaded the data.
Similarly, when the second box is checked, and Moderate is selected, a similar process takes place, but will only occur if more than 60% of the survey area has changed by more than 0.20m (0.66 ft).
When the second box is checked, and Lenient is selected, the survey will be flagged if more than 60% of the survey area has changed by 0.40m (1.31 ft).
Skip all QA
The survey will not be compared to the previous survey. Comparisons can be made by the user using a Volume Compare measurement.
Please note that this compare-to-past-surveys process is not mandatory, and can be unchecked by the user. This means we will not compare the current survey with previous ones, which will result in a quicker turnaround time.
The Slider will default to Lenient if the data provided in the uploader fails any initial quality checks. These checks include the following:
*This warning will force the slider to Lenient and not allow the user to change it.
If Lenient is recommended and the user opts to change the slider to Moderate or Strict, it is likely the survey will be placed on hold for investigation by our geospatial specialists resulting in a longer turnaround time.
More about skipping all QA
This option has the fastest turnaround time as it will send the survey through Propeller’s automated processing workflow. A computer will automatically mark aeroPoints and GCPs, and the survey will not be held for review by a GIS specialist for any reason.
Some GCPs may be excluded if they are not easily recognizable by a computer. We find the best results are achieved with AeroPoints or other checkerboard-style GCPs when placed on uniformly colored ground. However, the computer will still mark other styles of GCP markers. Please see our best practices for GCP markers in our article on How to Use Ground Control.
I still can't do it!
We wrote these articles to equip you with everything you need to get the job done on your own, but we understand that sometimes this isn't sufficient.
If you're stuck, you can connect with our support team by clicking the support button on the top right corner of your user portal.